Major update with company-stan, eldoc-stan, and flycheck-stan

Here is a screencast showing the new version in action.

This major update adds several new packages:

• company-stan for auto-completion
• eldoc-stan live help for function arguments
• flycheck-stan on-the-fly syntax checking with stanc (Version 2)

Two existing packages have been updated:

• stan-mode for indentation and syntax highlighting
  • Deprecate # comments while handling #include differently
• stan-snippets for function argument completion
  • Support | as an argument separator where appropriate

Other changes are:

• All packages include buttercup automated testing (test-*.el files) to define and ensure their behaviors.
• The minimum requirement for the Emacs version is 25.1.

TODOs:

• Plan for stanc version 3 support.